In order to optimise production, many attempts have been made within the oil industry to determine the flow properties, such as the volume flow rate, the hydro-carbon oil, water, and/or natural gas content in a well fluid, etc., of the fluid flowing in a casing down hole. The most common way of doing this is to take out samples above surface. However, logging tools able to determine the fluid properties have also been developed.
One example of a logging tool is shown in EP 0 372 598, in which two sets of eight electrodes are distributed around the circumference of the tool to be able to determine the fraction of gas in the oil and thus be able to determine the volume flow more accurately. In order to calculate the volume flow, the time between the measurement of substantially the same capacitance by the first and the second set of electrodes is calculated. Other electrodes in the form of guards are arranged between the electrodes. The guards are grounded to ensure that the electrical field is only distributed radially from the electrodes to the casing wall in each of the eight sections. These guards ensure that the measurements are independent of how the gas phase is distributed in the liquid phase, i.e. in the form of small bubbles, one large bubble, in the top of the casing, etc. Thus, only the fraction of gas in relation to the fraction of liquid is measured, and the capacitance measurements thus provide an average of the permittivity in one section. Subsequently, the fractions of gas measured in the eight capacitance measurements are used for determining the time between the measurement of substantially the same capacitance by the first and the second set of electrodes in order to estimate the volume flow.